A dissipator provides a continuously variable high-voltage load under control of a computer or other device capable of providing an analog control voltage. It is useful for discharging a capacitor such as a high voltage cable under test at a precisely-controlled rate. It is particularly useful as a component of a very low frequency high voltage test set to test a length of shielded power cable.
Known arrangements for discharging a capacitive load use multiple high voltage switches and resistors to attempt to dissipate power. However, there are operational difficulties associated with such known arrangements. They can only control the load in coarse steps. They do not provide control over a continuous analog range. Mechanical high voltage switches are prone to wear. Load control is inaccurate.
In order to overcome these operational difficulties, this patent document describes a novel arrangement for a dissipator. Rather than use switched resistors to dissipate power, there is provided a matrix of photo resistive cells, typically cadmium sulphide, arranged on a circuit board. These photo resistive cells are series connected and controlled optically to vary their resistances. An optical control signal is provided for each photo resistive cell by a corresponding light emitting diode (LED). The amount of light emitted by each LED is linearly controlled. Analog (linear) control is provided by an operational amplifier circuit that is driven by a control voltage of zero to 4 volts DC. This control voltage can be easily scaled to any value. It may also be digitally controlled. In the presently preferred embodiment, the dissipator can withstand up to 50 Kilovolts and will handle substantially 100 milliamps peak. It is not polarity sensitive and is easily scalable to higher or lower voltages and power levels. The output can be floating in reference to ground.